Effects of extracellular Mg2+ on transepithelial capacitance and Na+ transport in A6 cells under different osmotic conditions

The electrophysiological characteristics of monolayers of cultured renal ep ithelial A6 cells were studied under short-circuit conditions. Replacing ba solateral isosmotic (260 mOsm/kg H2O) media by hyposmotic (140 mOsm/kg H2O) solutions transiently increased the transepithelial capacitance (C-T) by 5 7.3 +/- 2.3% after 16 min. The transepithelial Na+ current (I-Na) increased concomitantly from 4.2 +/- 0.7 to 26.1 +/- 2.6 mu A/cm(2) with a time cour se that was noticeably slower, reaching its maximum after 60 min of hypoton icity. The transepithelial conductance (C-T) increased synchronously with I -Na. Analysis of blocker-induced noise in I-Na, using the amiloride analogu e 6-chloro-3,5-diaminopyrazine-2-carboxamide (CDPC), showed that the hypoto nic shock increased Na+ channel density (N-T) at the apical border. The pre sence of 10 mM Mg2+ on both sides of the epithelium suppressed the hypotoni city-induced C-T increase to 14.3 +/- 0.5%, whereas the I-Na increase was e ven larger than without Mg2+. Both effects of Mg2+ were located at an extra cellular, basolateral site. because apical administration was without effec t. whereas the acute basolateral addition of Mg2+ at the moment of the hypo tonic shock was sufficient. Interaction between Mg2+ and Ca2+ influenced th e behaviour of C-T. At constant osmolality (200 mOsm/kg H2O) 10 mM Mg2+ inc reased I-Na, leaving C-T unaffected, whereas 10 mM Ca2+ stimulated both I-N a and C-T. In the presence of mM Mg2+, however, the Ca2+-induced C-T increa se was abolished. The failure of C-T to increase during stimulation of I-Na by Mg2+ suggests that the divalent cation activates pre-existing channels in the apical membrane. Noise analysis showed that the natriferic effects o f Mg2+ were also mediated by an increase in N-T. The moderate initial incre ase in C-T in the presence of Mg2+ under hypotonic conditions, occur-ring i n parallel with increases in G(T) and I-Na reflects most lihrly Na+ channel insertion induced by the hypotonic treatment. However, the large, transien t, Mg2+-sensitive increase in C-T, not correlated with increases in G(T) an d I-Na, seems to be unrelated to Na+ channel recruitment.